<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.4.1/jquery.min.js"></script>
    <script>
        $(function() {

            // $("li:first").css("color", "red");
            //$("li:last").css("color", "red");
            //$("li:even").css("color", "red");
            //$("li:odd").css("color", "red");

            //$("li:eq(2)").css("color", "red");
            //$("li:gt(2)").css("color", "red");
            //$("li:lt(2)").css("color", "red");
            //$("li:not(:eq(2))").css("color", "red");
            //$("li:not(:odd)").css("color", "red");
            //$("li>:header").css("color", "red");
            $("input").focus();
            $(":focus").css("background-color", "yellow");
        });
    </script>
    <style>

    </style>
</head>

<body>
    <form>
        <input></input>
    </form>
    <ul>
        <li>收到货发</li>
        <li>收到货发</li>
        <li>收到货发</li>
        <li>收到货发</li>
        <li>收到货发
            <h6>水电费水电费</h6>
        </li>
    </ul>
    <h2>撒的发生</h2>
    <h3>撒的发生</h3>
    <h4>撒的发生</h4>
</body>

</html>